a toe in the door

The chance to do something that could lead to further opportunities. The phrase is often used to refer to employment. An internship might not sound very interesting, but it's a great way to get a toe in the door at this company. I knew that I could prove myself to be a valuable asset to the company, if only I could get a toe in the door. Good luck getting a toe in the door in Hollywood!

a toe in the door

a (first) chance of ultimately achieving what you want; a position from which further progress is possible. informal

The image here is of placing your foot in a doorway in such a way as to prevent the door being closed in your face.